Communication networks are being developed in which traffic is carried within packets each of which is routed to an appropriate destination on the basis of information carried in a header attached to that packet. Such networks comprise a number of nodes each of which has a routing table to which reference is made to determine the processing of incoming packets. These networks include asynchronous transfer mode (ATM) and Internet Protocol (IP) networks.
A development of this packet transport technique provides communication across two or more effectively independent networks, generally referred to as an autonomous system, each with its own set of nodes and routing tables. In such an arrangement, autonomous system border routers (ASBR) are provided at the boundary of each autonomous system so as to provide communication paths between these systems or networks. The border routers also provide a means of passing information between the customers or networks so that the packets from one network can be correctly routed to destinations in another.
In such an arrangement, routing information is passed between the border routers using a protocol that is generally referenced to as a border gateway protocol (BGP). This protocol permits the border routers to exchange routing information concerning destinations that can be reached from these routers. The border routers may be referred to as BGP peers as they share a peering relationship across a BGP connection.
A typical arrangement of this type is shown in FIG. 1 of the accompanying drawings which is introduced for explanatory and comparative purposes. This arrangement comprises two networks or autonomous systems 11a, 11b having respective edge routers A, D providing a communications path therebetween. Router A in autonomous system 11a is a BGP (border gateway protocol) peer with router D and has route information to access nodes B and C in system 11a. Similarly, router D is a BGP peer with router A and has route information to access nodes E and F in system 11b. Thus, router A advertises itself to D as the next hop router on the route to B and C. Similarly, router D advertises itself to A as the next hop router on the router to E and F. This use of border gateway protocol (BGP) is often referred to as exterior BGP (EBGP), as route information is disseminated to a router that is exterior to the system to which that route information relates.
Within the network 11a, router A disseminates its route information, received from its BGP peer D, to routers B and C, again using BGP (border gateway protocol). The analogous process is performed by router D in network 11b. This is generally referred to as interior BGP (IBGP).
A recent development in network technology has been the introduction of multiprotocol label switched (MPLS) networks. In such networks, a label distribution protocol (LDP) provides a route distribution mechanism for routing packets across an autonomous system. If a packet is destined for another autonomous system, the LDP (label distribution protocol) sets up a label mapping for a route to the correct next hop border router. The packet must then be re-labelled according to the protocol of the new autonomous system using the destination IP address in its IP header.
The above process requires examination of the packet IP header at the boundary of the two autonomous systems or networks. This adds to the complexity of the system and negates some of the benefits that have been provided by the introduction of MPLS. One approach to this problem is the use of an explicit routing mechanism to define a label switched path (LSP). Using this mechanism, it is possible to pre-establish labels for an end-to-end label switched path from a specified source to a specified destination such that no reprocessing of the IP header is necessary. This however, leads to statically provisioned paths between autonomous systems and does not address the problem of determining dynamic established paths e.g. to enable load balancing and to facilitate traffic engineering.